as for me, the real question is: could Gortash be an actual ally/party member? That's a bad boy I am really curious about (and would be sad to kill)!
I'm inclined to think neither of the avatars of the dead three can become companions, even if it makes me a bit sad because I'd always be recruiting Orin as a companion.
Now Larian did tweet this yesterday through the official BG3 twitter account in relation to Orin: "No, you can’t fix her. But… do try! What could go wrong? 🙂" I'm guessing the same applies to Gortash. But damn it I will try everything I can to recruit Orin.